This application makes reference to, incorporates the same herein, and claims all rights accruing thereto under 35 U.S.C. 119 through our patent application entitled Charge Roller For A Developing Device Of An Image Forming Apparatus And Method For Fabricating The Same And Tool For Fabricating The Charge Roller earlier filed in the Korean Industrial Property Office on the day of Nov. 29, 1999 and there duly assigned Ser. No. 1999/53498.
1. Field of the Invention
The present invention relates to a charge roller of a developer for use of an image forming apparatus adopting an electrophotographic printing system, and more particularly, to a charge roller of a developer for an image forming apparatus in which a semiconductive resilient rubber layer is formed of a double layer constituted by a foam rubber layer and a non-foam rubber layer, or a single layer of a cross linking rubber layer, to thereby minimize volume resistivity of the semiconductive resilient rubber layer, maximize crosslinking ratio of molecule of rubber, and suppress increase of durability of the semiconductive resilient rubber layer. Further, the present invention relates to a method of manufacturing such a charge roller and a receptor device employed for such a method.
2. Background Art
In general, an image forming apparatus adopting an electrophotographic printing system, for example, laser printer, facsimile apparatus, copier or similar image forming apparatus, is provided with a developer having a photosensitive drum and a developing roller. A photosensitive drum is formed with at an outer periphery thereof an electrostatic latent image via an exposer, and supplied with a toner via an adjacent developing roller, and develops the electrostatic latent image into a visual image using a toner, thereby forming an image onto a recording medium.
A conventional developer for an image forming apparatus is disclosed in U.S. Pat. No. 5,132,734 xe2x80x9cDeveloping apparatusxe2x80x9d, U.S. Pat. No. 5,260,748 xe2x80x9cElectrostatic image developer dispenserxe2x80x9d, U.S. Pat. No. xe2x80x9cDeveloping device for an image forming apparatusxe2x80x9d, U.S. Pat. No. 5,771,426 xe2x80x9cDeveloping device using a toner and carrier mixturexe2x80x9d, and U.S. Pat. No. 5,787,328 xe2x80x9cRotary developing device for an image forming apparatusxe2x80x9d. 
In such a conventional developer for an image forming apparatus, a charge roller is generally disposed in the vicinity of a photosensitive drum so as to rotate at a constant speed, being engaged with the photosensitive drum. Here, if the charge roller is applied with a voltage having a predetermined size, so-called xe2x80x9cPaschen dischargexe2x80x9d occurs, and the surface of the photosensitive drum which is in contact with the charge roller is electrostatically charged according to a relation among resistance and surface state of the charge roller and the voltage applied.
A conventional charge roller is disclosed in U.S. Pat. No. 5,499,078 xe2x80x9cCharge roller and image forming apparatus using the samexe2x80x9d, U.S. Pat. No.5,600,414 xe2x80x9cCharging roller with blended ceramic layerxe2x80x9d, U.S. Pat. No. 5,768,653 xe2x80x9cElectrophotographic printing device with a charging rollerxe2x80x9d, U.S. Pat. No. 5,792,533 xe2x80x9cElectrostatic charging rollerxe2x80x9d, and U.S. Pat. No. 5,852,758 xe2x80x9cCharge roller displacement mechanismxe2x80x9d. 
A conventional charge roller has a configuration in that a metallic rod has an outer periphery pressedly coated with a semiconductive resilient rubber with a low durability so that a photosensitive drum may have a uniformly charged surface.
Recently, it has been established through a variety of studies on the structure of a charge roller that a semiconductive resilient rubber should maintain a volume resistivity of 107 xcexa9 cm to 108 xcexa9 cm to allow a photosensitive drum to have a uniformly charged surface contacting a charge roller.
However, it is extremely difficult to produce a charge roller with an excellent quality unless an additional treatment is conducted to a semiconductive resilient rubber since a semiconductive resilient rubber constituting an outer surface of a charge roller has its own volume resistivity of 108 xcexa9 cm or higher.
If a charge roller with a volume resistivity of 108 xcexa9 cm or higher contacts a photosensitive drum, an excessively high voltage is required for charging the surface of the photosensitive drum. Thus, an amount of ozone generated by the applied high voltage significantly increases, resulting in a serious environmental pollution.
Considering such characteristics of semiconductive resilient rubber, a variety of techniques for reducing a volume resistivity of a semiconductive resilient rubber are sought in a conventional system.
For example, U.S. Pat. No. 5,637,395 xe2x80x9cPowder coated charge rollerxe2x80x9d discloses a method of adding an additive like a conductive carbon powder or alkali metal salt to a semiconductive resilient rubber so as to reduce a volume resistivity of the semiconductive resilient rubber.
Another example shows a method of replacing a semiconductive resilient rubber by an acrylonitrile butadiene rubber or epichlorohydrin rubber.
In addition to the above-mentioned approaches for reducing volume resistivity of a semiconductive resilient rubber, some different approaches have been proposed for enhancing characteristics of a semiconductive resilient rubber.
For example, U.S. Pat. No. 5,497,219 xe2x80x9cCharge rollers having improved layer structure and/or surface characteristics in an image forming apparatusxe2x80x9d and U.S. Pat. No. 5,786,091 xe2x80x9cCharge roller for an image forming apparatusxe2x80x9d disclose a method of coating a surface of a semiconductive resilient rubber with polyamide, fluoric resin, or epichlorohydrin rubber liquid utilizing a spraying or dipping system so that the semiconductive resilient rubber may have a desirable thickness and surface roughness.
U.S. Pat. No. 5,248,560 xe2x80x9cFilled urethane developer rollerxe2x80x9d describes a method of replacing a semiconductive resilient rubber by a polyurethane rubber having superior abrasion resistance and electrical characteristic.
However, many problems arise from those known approaches when applied to practice, as follows.
First, with the above-mentioned method disclosed in U.S. Pat. No. 5,637,395, it is extremely difficult to uniformly disperse an additive like a conductive carbon powder or alkali metal salt onto a semiconductive resilient rubber.
In this case, since the uniformity of the additive applied to the semiconductive resilient rubber is extremely poor, an overall volume resistivity of the semiconductive resilient rubber becomes non-uniform. Accordingly, an overall surface of a photosensitive drum contacting the semiconductive resilient rubber is non-uniformly charged, resulting in an undesirable image being produced.
In case where an additive is added, the durability of a semiconductive resilient rubber rapidly increases, and it will be difficult to maintain the durability of the semiconductive resilient rubber at 40 or less as prescribed by JISA (Japanese Industrial Standards type AK6301). Moreover, if a diameter of a charge roller is reduced at such a state, it will be extremely difficult to ensure a uniform contact between the charge roller and the photosensitive drum. As a result, it will be difficult to reduce the size of the charge roller.
The approaches presented in U.S. Pat. Nos. 5,497,219 and 5,786,091 require a coating process utilizing coating liquid like polyamide, fluoric resin, or epichlorohydrin rubber in addition to a process of forming a semiconductive resilient rubber at an outer periphery of a metal rod, thus significantly deteriorating an overall product process efficiency.
Moreover, the process of coating such liquid makes it difficult to maintain at a low durability of the semiconductive resilient rubber similarly to the case of adding an additive. This will also make it difficult to reduce a size of a charge roller.
The approach disclosed in U.S. Pat. No. 5,248,560 allows a relatively higher quality charge roller to be produced, it is still problematic from the overall production cost aspect since a high price polyurethane is employed for this approach.
If a semiconductive resilient rubber is replaced by an acrylonitrile butadiene rubber or epichlorohydrin rubber having a low volume resistivity, the completed product may have an outer rubber surface having a volume resistivity suppressed down to a level lower than a predetermined level. However, it will be extremely difficult to maintain the durability of the outer rubber surface at 10 or less as prescribed by JISA. Moreover, if a diameter of a charge roller is reduced at such a state, it will be extremely difficult to ensure a uniform contact between the charge roller and the photosensitive drum. As a result, it will be difficult to reduce the size of the charge roller.
In addition, such a polar synthetic rubber like an epichlorohydrin rubber has characteristics where a large amount of non-crosslinked low molecular polymeric substance exists at a surface of the rubber. Therefore, if a charge roller having a rubber outer surface made of such polar synthetic rubber is in contact with a photosensitive drum during stoppage of operation of image forming apparatus, the low molecular polymeric substance constituting the charge roller is likely to be migrated to a surface of the photosensitive drum. As a result, an abnormal phenomenon may occur in that an image may not be formed onto the surface of the photosensitive drum.
As a method of suppressing such an abnormal phenomenon, efforts have been made to maximize the crosslinkage density of the outer rubber surface. However, this method still increases durability of outer rubber surface while reducing the above-mentioned abnormal phenomenon, and it will be difficult to reduce the size of the charge roller.
Therefore, it is an object of the present invention to uniformly charge a surface of a photosensitive drum contacting a resilient rubber while maintaining a volume resistivity of the resilient rubber constituting outer surface of a metal rod at 107 xcexa9 cm to 108 xcexa9 cm.
It is another object of the present invention to uniformly maintain volume resistivity of a resilient rubber at 107 xcexa9 cm to 108 xcexa9 cm while simplifying an overall product process.
It is still another object of the present invention to minimize voltage required for a charge roller to charge a surface of a photosensitive drum while maintaining a volume resistivity of a resilient rubber at 107 xcexa9 cm to 108 xcexa9 cm.
It is further still another object of the present invention to significantly reduce an amount of ozone by minimizing a charge voltage of a charge roller required for charging a surface of a photosensitive drum.
It is further still another object of the present invention to achieve a uniform image while maintaining uniform charged state of a photosensitive drum.
It is further still another object of the present invention to reduce a size of a charge roller while minimizing durability of a resilient rubber constituting an outer surface of a metal rod.
It is further still another object of the present invention to prevent an abnormal phenomenon from being occurred at a surface of a photosensitive drum even if a charge roller contacts for a long time period during a stoppage of operation of an image forming apparatus while minimizing a non-crosslinked low molecular polymeric substance existing at a resilient rubber.
It is still further an object of the present invention to enhance the quality of printing image by allowing an image composition for OA (office automated) machines, such as ink or toner to have metallic gloss revelation capability using organic synthesis metallic material instead of inorganic pigment.
To accomplish the above objects, there is provided a receptor device for an exclusive use in manufacturing a charge roller, the receptor device including a pipe-shaped main body having both ends opened so as to define a receipt space, and a pair of plugs inserted into the both ends of the main body for sealing the receipt space. Here, the main body of the receptor device is made of high molecular polymeric substance with a low thermal conductivity.
A metal rod pressedly-coated with a foam rubber consisting of epichlorohydrin oxide rubber, acrylonitrile butadiene rubber, filler, cross linking agent, and a blowing agent is loaded into the receipt space of the receptor device. Subsequently, the receptor device loaded with the metal rod is loaded again into a dry heating furnace, and the foam rubber is heated at a temperature of 130 C. to 150xc2x0 C.
When such a heating process is performed for a selected time period, for example, 15 minutes to 25 minutes, the foam rubber is blown by the blowing agent thus filling the receipt space of the receptor device. Here, since the main body of the receptor device is made of a polymeric substance with a low thermal conductivity, an outer surface of the foamed rubber contacting an inner wall of the receipt space becomes a semiconductive non-foamed rubber layer while an inner surface of the foamed rubber which does not contact the inner wall of the receipt space becomes a semiconductive foamed rubber layer.
Subsequently, the main body of the receptor device is disassembled, and the metal rod coated with the foam rubber substance is transported outside and loaded into a dry heating furnace so as to heat the foam rubber substance at a selected temperature, for example, 130xc2x0 C. to 150xc2x0 C.
Then, the metal rod pressedly-coated with foam rubber substance is cut into a selected width, thereby completing a process of producing a charge roller of a developer for use in an image forming apparatus. The charge roller produced according to an embodiment of the present invention consists of a metal rod, a semiconductive foamed rubber layer surrounding an outer periphery of the metal rod, and a semiconductive non-foamed rubber layer surrounding an outer periphery of the semiconductive foamed rubber layer.
In the present invention, the rubber layer constituting an outer periphery of the metal rod may have a volume resistivity of 107 xcexa9 cm to 108 xcexa9 cm owing to an action of the epichlorohydrin oxide rubber and acrylonitrile butadiene rubber. Therefore, the charge roller produced according to the process of the present invention is capable of sufficiently charging a photosensitive drum utilizing a small sized charging voltage.
Meanwhile, a charge roller of a developer for use of an image forming apparatus can be produced in accordance with another embodiment of the present invention. If such is a case, a compression molding machine is employed for coating an outer periphery of a metal rod with a crosslinked rubber substance consisting of epichlorohydrin oxide rubber, acrylonitrile butadiene rubber, and a cross linking agent. Here, the cross linking agent is made up of a peroxide consisting of dicumyl peroxide and benzoyl peroxide.
Subsequently, the metal rod coated with the crosslinked rubber substance is loaded into a dry heating furnace so as to heat the crosslinked rubber substance at a selected temperature, say, 140xc2x0 C. to 150xc2x0 C. If such a heating process is performed for a selected time period, for example, 55 minutes to 65 minutes, the epichlorohydrin oxide rubber and acrylonitrile butadiene rubber contained in the crosslinked rubber substance form a peroxide crosslinkage due to an action of the cross linking agent.
Subsequently, the heated crosslinked rubber substance is heated again at a selected temperature, for example, 100xc2x0 C. to 120xc2x0 C. After performing such a heating process for a selected time period, say, 4 hours to 12 hours, the residual crosslinking agent which did not join the crosslinking process is completely removed.
Then, the metal rod pressedly-coated with the crosslinked rubber substance is cut into a predetermined width, thereby completing a process of producing a charge roller of a developer for use of an image forming apparatus. A charge roller produced according to an embodiment of the present invention consists of a metal rod and a crosslinked rubber layer surrounding an outer periphery of the metal rod.
In another embodiment of the present invention, the rubber layer constituting an outer periphery of the metal rod may have a volume resistivity of 107 xcexa9 cm to 108 xcexa9 cm owing to an action of the epichlorohydrin oxide rubber and acrylonitrile butadiene rubber. Therefore, the charge roller produced according to the process of the present invention is capable of sufficiently charging a photosensitive drum utilizing a small sized charging voltage.
The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.